The Chemistry of ... Glue

Biochemists turn to mussels for a real bonding experience

Nothing quite beats the intransigence of a mussel. From glands in its sluglike foot, the animal secretes a glue that in less than five minutes hardens into a filament, or byssal thread, that will tether it for life to an intertidal rock. Within a few days, it has anchored its shell by a cable of several hundred such threads that will withstand years of pounding surf. Mussel glue can resist forces of a thousand pounds per square inch. Mussels can stick to Teflon.

"I've gained an enormous respect for these creatures," Herbert Waite, a marine biochemist at the University of California at Santa Barbara, says. "They live in an environment of turbulence. They can't afford to make something flimsy."

Humans, alas, can and do. Our superglues, Krazy Glues, and specialized industrial glues work spectacularly well on dry land. Submerged, however, they pale in comparison with the mussel's product. "There's no glue that can do that underwater," Waite says. "If you went to the store, bought epoxy, and mixed it underwater, it would be a flimsy bond." For years researchers have sought in vain to mimic nature and create a glue that will bond quickly and firmly when wet; surgeons and naval repairmen, among others, are eager for such a product. At last the tide is turning. Thanks in part to years of research by Waite, a synthetic, pseudo-mussel glue appears to be close at hand.

The effort to understand marine glues began in the 1960s and initially was focused on the stickiness of barnacles. Those crustaceans have plagued navigators since Roman times, fouling the hulls of ships and slowing travel considerably. Today the U.S. Navy spends more than $5 million each year researching "biofoulers"—barnacles, mussels, and various marine slimes—yet, despite great effort, the secret of the barnacle's glue remains largely hidden beneath the animal's tough, tiny shell.

"They're playing a really tight hand of poker," Waite says of the barnacles. "All the mechanisms of making their holdfast are hidden under their base, which is quite small. Plus, if you detach the organism to study it, you kill it."

Blue mussels have offered scientists an easier and more fruitful course of study. A mussel is a bivalve mollusk, a goop of living innards enclosed in a hinged shell. Because the byssal threads are external to both the animal's body and the shell, the glue chemistry is simpler to access and analyze. Moreover, each newly made thread is less solid than the previous one, affording a lesson in how its chemistry changes over time.

Over the years, Waite—"Mr. Mussel," to colleagues—has done more than any single biochemist to decipher the chemistry of blue-mussel glue. Although many of the specifics are still unclear, Waite has found that the overall mechanics are akin to those of the two-part epoxy glues available in hardware stores. The mussel's glue gland comprises two separate compartments: One produces resinlike proteins, and a second produces chemicals that behave like hardeners. On entering the water, the mussel adhesive proteins—MAPs, as they're called—mix and, in minutes, cure.

"Curing, or cross-linking, refers to the establishment of bonds or contacts between polymers," Waite says. Protein polymers are long arrangements of molecules; think of them as spaghetti noodles. Now recall what happens to spaghetti noodles when you cook them, Waite says. If you're careless and simply drop the noodles into the pot in a clump, without first separating them, you'll wind up with thick, wet ropes of pasta. That's sort of what happens when mussel adhesive proteins cross-link. "Because of their proximity, the polymers begin to make contact with one another. Eventually, you can't pull on one without pulling on several at a time."

Cross-linking isn't everything; too much of it can produce an adhesive as brittle as glass. As it turns out, mussel adhesive proteins contain an assortment of five to 10 different "flavors" of polymer noodle, each with a slightly different role. "Some might be catalysts that speed up the cross-linking process," Waite says. "Others work to separate the noodles slightly. There are all kinds of subtleties in trying to make a robust glue."

Some of the key noodles, Waite discovered, contain the amino acid dopa (short for L-3, 4-dihydroxyphenylalanine). Dopa is more commonly known as a neurotransmitter and a major ingredient in current treatments for Parkinson's disease. Its role in mussel glue came as something of a surprise. "It isn't found in high concentrations in other proteins except for mussel adhesive proteins," says Phillip Messersmith, a biomedical engineer at Northwestern University.

Messersmith first came across Waite's research several years ago, when he was an assistant professor at Northwestern's school of dentistry. The dental community has a tremendous interest in mussel adhesives, he says. Reconstructive dental surgery demands a glue that can be applied while still soft, bonds quickly in a wet environment, and adheres strongly to mineralized tissues that contain metal oxides (teeth and bones). "The current generation of binding agents really is not good at bonding on those kinds of surfaces," Messersmith says. Mussel adhesive proteins, in contrast, are superb at clinging to wet, mineralized surfaces (rocks).

Building on Waite's research into blue mussels, Messersmith and his colleagues are developing a mussel-like glue that they believe is nearly equal to the real thing. They attach dopa, the critical component in mussel adhesive proteins, to compatible, laboratory-made polymers, creating gooey, gluey gels that cross-link and bond much as real mussel adhesive proteins do, even in wet conditions. The gels stick firmly to a wide range of surfaces, including glass, metals, metal oxides, and semiconductors. If the gels prove nontoxic and prompt no immune response, they will be ideal candidates for use in surgery and other biomedical tasks.

Messersmith envisions a wide variety of uses for the gels. Injected into the body, they could administer drugs to the lungs, ovaries, and other specific, hard-to-reach areas of the human interior. For that matter, pseudo-mussel glue could prove useful anywhere tissues need to meet and stay met. "We hope to be able to attach our devices equally well to hard tissues and soft tissues," Messersmith says. "The connection of tendons and ligaments is one very important area that we hope to make a contribution in."

Paradoxically, Messersmith has also found a way to put sticky dopa polymers to precisely the opposite use, as a kind of anti-glue. Essentially, he has created a spaghetti noodle with a sticky end and a repellent end. The sticky end, the dopa portion, adheres to a surface—pretty much any surface—and is hidden by the rest of the noodle. Most of the noodle, meanwhile, the part that greets the world, is made up of a well-known nonadhesive polymer called polyethylene glycol. The result is a surface, anchored by dopa, that prohibits cells—and potentially live organisms—from sticking to it. "All similar work to date has been to develop polymers that are sticky. What we've done is to turn things upside down by utilizing the adhesiveness of dopa to convert surfaces from fouling to non-fouling."

Messersmith says that the shipping industry is most likely to express an interest in his antifouling research. Traditionally, the war against fouling organisms has been waged with toxic paints, high-pressure hoses, and old-fashioned scrubbing devices. A hull coated with dopa-laced polymers instead would essentially turn the enemy's weapon against itself. "It would use the natural fouling properties of mussels to prevent them from attaching," Messersmith says. No wonder mussels are so tight-lipped with their secrets.